Since the adoption of ITU/CCIR Recommendation 601 the digital processing of television signals has almost always been carried out with an orthogonal sampling structure. However, there are applications in which it is difficult to convert the signal into this form; a well-known example is the processing of the output of a domestic video cassette recorder. In this case the instability of the line frequency makes it difficult to derive an orthogonal sampling grid.
In order to sample an analogue television signal orthogonally, it is necessary to derive a repetitive sampling clock signal which defines the instants at which the signal is to be digitised. If the signal were stable these instants would be regular, and the clock would be an integral multiple of the signal's line frequency. In the case of an unstable signal the timing of the clock pulses must vary, so that the same number of samples is taken on every line, even though all lines are not of the same duration.
One known method of doing this is to use a high-bandwidth (of the order of 1 khz), frequency-agile phase locked loop to derive the clock. The clock oscillator attempts to follow the signal's varying line frequency and so maintain the same number of samples on every line. This is difficult to achieve; one problem is that the wide-bandwidth loop is more susceptible to noise than a narrow-band loop would be. Another problem is that if the loop is capable of operating over a wide frequency range it is less able to phase-lock accurately because of practical limitations to the resolution of the control signal.
A second known method is to sample the signal with a stable, free-running sampling clock and then process the resulting non-orthogonal samples to obtain orthogonal samples. This is done by the horizontal interpolation of new sample values from the actual samples taken. This interpolation corresponds to a filtering process which modifies the frequency response of the sampled signal; if a flat response is required a large number of input samples need to be used to generate each interpolated output sample.